In recent years, a compact camera and a cellular phone with camera function are required to be miniaturized. Accordingly, an electric flash device used as an artificial light source is also required to be miniaturized.
A structure needs to be simplified to cope with miniaturization. In order to simplify a structure of a light-emitting unit of an electric flash device in which a reflective umbrella used in a conventional electric flash unit is eliminated and a reflective film is formed on an outer peripheral surface of an electric discharge tube is proposed (see Patent Document 1).
FIG. 10 is a sectional view showing an electric discharge tube used in a conventional electric flash device. In the electric flash device, reflective film 94 is formed on an outer peripheral surface of cylindrical glass bulb 92 serving as a constituent element of electric discharge tube 91 except for a belt-like (in FIG. 10, in a direction from the front surface of the plane of the drawing to the rear surface thereof) transparent portion 93 that transmits light. Reflective film 94 is formed by depositing a metal such as aluminum, silver or the like, and functions as a reflective umbrella that reflects light emitted in electric discharge tube 91.
However, since light emission in electric discharge tube 91 occurs by light emission of an entire gas excited in electric discharge tube 91, as shown in FIG. 10, light is not reflected by reflective film 94 at point L, and, as indicated by arrow D, the light is irradiated behind electric discharge tube 91 in vain. In order to reduce the light volume loss and to reflect light ahead of electric discharge tube 91, reflective film 94 formed in electric discharge tube 91 needs to be formed at a deposition angle of 180° or more in a circumferential direction on an outer peripheral surface in a more proper range to have a sufficient thickness.
When a reflective film obtained by metal deposition is to be formed by a conventional depositing method on an outer peripheral surface of cylindrical electric discharge tube 91 in a direction indicated by arrow Y, a deposition material easily adheres to outer peripheral surface 95 facing a deposition source. However, a deposition material does not easily adhere to outer peripheral surface 96 (semi-peripheral surface being far from the deposition source on a tube surface) that is hidden behind the deposition source, and reflective film 96 becomes extremely thin. For this reason, light emitted in electric discharge tube 91 cannot be sufficiently reflected.
However, a light emitting device such as an electric flash (strobe) device using a conventional electric discharge tube cannot obtain a uniform light intensity distribution because a light value is short.